Apparatus for signal transmission and reception between camera and attachment

ABSTRACT

The number of terminals between a camera and an attachment can be reduced by a new combination of camera and attachment. The combination comprises first and second terminals for connecting the attachment to the camera; an information signal generating circuit on either one of the camera or the attachment for generating in time series first and second information signals and for transmitting said first and second signals to the other one through the first terminal; first and second responding circuits on the other one and operable in response to the first and second information signals respectively; a switchover signal generating circuit provided on the one for generating a switchover signal and for transmitting the signal to the other one through the second terminal; and a switchover circuit for transmitting the first and second information signals coming through the first terminal to the first and second responding circuits selectively in accordance with the switchover signal coming through the second terminal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to apparatus for signal transmission and reception and more particularly to a signal transmission and reception apparatus between a camera and its attachment such as an electronic flash unit.

2. Description of Prior Art

In recent years there has been an increasing tendency to transmit and receive many signals between a camera and its attachment such as electronic flash unit. Probably, the most reliable method of transmitting and receiving so many signals without interference is to provide the same number of terminals as the number of the signals between the camera and its attachment. However, this method is not preferable in view of the space required for such many terminals.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide a signal transmission and reception apparatus which enables two or more different signals to be transmitted through the same terminal without interference between a camera and its attachment.

It is another object of the invention to provide an electronic flash unit which is able to receive from the camera two or more different signals through one and the same terminal without interference.

Other and further objects, features and advantages of the invention will appear more fully from reading the following description taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an electrical circuit of a camera provided with a signal transmission and reception apparatus according to the invention;

FIGS. 2 and 3 respectively show different embodiments of the electronic flash unit provided with the signal transmission and reception apparatus according to the invention and adapted to be mounted on the camera shown in FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

A preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

For purposes of this specification, the terms "electrical shutter control mode", "mechanical shutter control mode", "flashlight photographing mode", "TTL (through the lens) flash output control mode", "camera interlocking external flash output control mode" and "independent external flash output control mode" as used herein are defined as follows:

Electrical shutter control mode: a mode in which the shutter time is electrically controlled.

Mechanical shutter control mode: a mode in which the shutter time is mechanically controlled by means of a mechanical governor.

Flashlight photographing mode: a mode in which the shutter is automatically set to a flash synchronous time according to the signal from the electroflash device.

TTL flash output control mode: a mode in which the flashlight reflected by the object and transmitted through the objective lens of the camera is measured by a photo-detector mounted in the camera and the flashlight emission of the flash device is terminated by a signal dependent on the flash output measured.

Camera interlocking external flash output control mode: a mode in which various exposure factors such as film sensitivity and diaphragm aperture then set on the camera's side are introduced into the electroflash device from the camera and the flash light emission is terminated depending on these exposure factors as well as the photometric output of the photo-detector on the side of electroflash device.

Independent external flash output control mode: a mode in which data of exposure factors such as film sensitivity and diaphragm aperture are introduced directly into the flash device and the flashlight emission is terminated depending on these given data as well as the photometric output of the photo-detector on the side of the electroflash device.

By the term "flash synchronous time" as used herein is meant such shutter time which is the maximum speed or a little lower speed than the maximum among all shutter times at which the shutter can be completely opened.

FIG. 1 shows an electric circuit used in a single lens reflex camera according to the invention.

1 is a power source on the camera's side. A mode change-over switch SW₁ is series-connected to the power source 1. The mode change-over switch SW₁ is connected with a mode change-over member not shown. When the mode change-over member is moved to set the electrical shutter control mode, the switch SW₁ is closed. When the mechanical shutter control mode is set by the mode change-over member, the switch SW₁ is opened in link with the movement of the member.

SW₂ is a power supply switch interlocked with a shutter release button not shown. When the shutter release button is pushed down over the first half stroke, the power supply switch SW₂ is closed. By closing the switch SW₂ a power source timer circuit composed of transistors Q₁ and Q₂, resistor R₁ and capacitor C₁ is brought into operation. Power supply to the circuit on the camera's side is effected through the transistor Q₁ which is turned on by closing the power supply switch SW₂. When the switch SW₂ is opened, charging of the capacitor C₁ is started. The transistor Q₁ can remain conductive, namely On for a time determined by the time constant of capacitor C₁ and resistor R₁. After this predetermined holding time has been passed, the transistor Q₁ becomes off. In this manner, the power source timer circuit Q₁, Q₂, R₁, C₁ supplies power to the circuit in the camera not only during the switch SW₂ being closed but also continues the power supply for the predetermined constant time after the switch SW₂ is opened so long as the change-over switch SW₁ is in the closed position. A temperature-compensated constant current from a constant current source 2 flows into a variable resistor R₂. The voltage produced at the both ends of the variable resistor R₂ has a characteristic which is proportional to absolute temperature. The resistance value of this variable resistor R₂ is variable depending on the set value of film sensitivity and produces a voltage corresponding to Apex value of film sensitivity. This film sensitivity voltage is transmitted to a control circuit 3 through a follower amplifier A₁. The control circuit 3 has various known functions such as shutter priority automatic exposure control, diaphragm priority automatic exposure control, programmed automatic exposure control, manual exposure control, automatic change-over of the shutter time to flash synchronous time for flashlight photographing mode, display of controlled exposure value, electromagnetic releasing etc. A variable resistor R₃ is interlocked with a shutter dial not shown. The function of the variable resistor R₃ is to introduce into the control circuit 3 the shutter time manually set for shutter priority automatic exposure control or manual exposure control. R₄ is a variable resistor interlocked with the setting motion of diaphragm aperture of the taking lens. The resistance value of variable resistor R₅ corresponds to the fully open, namely minimum F-number of the taking lens.

PD₁ is a photo-detector which is so disposed that it can receive the light transmitted through the camera lens when the quick return mirror in the camera is in its lowered position and it can not receive the light when the mirror is moved upward. The outputs from these elements R₄, R₅ and PD₁ are all introduced into the control circuit 3. SW₃ is a release switch which is closed when the shutter release button is pushed down over the second half stroke subsequent to the above mentioned first half stroke. In response to the closing of this switch SW₃, the control circuit 3 drives a release magnet Mg1. With this driving of the release magnet, a series of exposure operations well known in the art of single-lens reflex camera are initiated. For instance, stopping down of the diaphragm, rising of the quick return mirror, opening of the shutter etc. are successively performed.

SW₄ is a memory switch interlocked with the shutter releasing motion. The memory switch SW₄ is closed immediately before the incident light to the photo-detector PD₁ is blocked and opened when the blockage against the incident light is removed. During the time when the memory switch SW₄ is in the closed position, a memory circuit within the control circuit 3 memorizes the output value of the object illuminance from the photo-detector PD₁ at the closing of the switch or the computed exposure value resulting from the object illuminance and values of other exposure factors. For the above described automatic exposure control, the exposure control is carried out on the basis of this stored value in the memory.

Mg2 is a magnet for controlling the shutter closing operation. For electrical shutter control mode, the magnet Mg2 causes the shutter to start closing by an output from the control circuit 3. Mg3 is a diaphragm controlling magnet which controls the aperture diameter of the diaphragm for the taking lens.

Constant current source 4, resistors R₆, R₇, R₈, R₉, transistors Q₃ and Q₄, diode D₁ and operational amplifier A₄ constitute together a reference voltage source stable against variations in temperature and source voltage. The manner of operation of this reference voltage source is as follows:

The current of constant current source 4 is divided into transistors Q₃ and Q₄ in accordance with the ratio of resistance R₆ to R₇ owing to the negative feedback operation of the operational amplifier A₄. Making use of the fact that the voltage at the base-emitter stage of transistors Q₃ and Q₄ depends on the current density at the junction between base and emitter, there is produced between the connection point of resistor R₉ and diode D₁ and the positive line of the power source 1 a voltage having a positive temperature coefficient proportional to the absolute temperature. This voltage cancels out the negative temperature coefficient of the voltage produced at the diode D₁. At the output of the operational amplifier A₄, therefore, a stable reference voltage is obtained the temperature coefficient of which is zero. In the case where in the diode D₁ is a silicon diode, the reference voltage output is in the order of 1.2 V. Connected to the output terminal of the reference voltage source, that is, the output terminal of the operational amplifier A₄ is the (+) input namely the noninverting terminal, of a battery checking comparator A₂ whose (-) input, namely the inverting terminal, is connected to the connection point of voltage dividing resistors R₁₀ and R₁₁ for dividing the voltage of the power source 1. The output of the comparator A₂ is at high level (High) when the voltage of the power source 1 is at a level sufficient enough to normally operate the camera. When the voltage of the power source 1 is lower than the level, the output of the battery checking comparator A₂ becomes low level (Low). When the output of the comparator A₂ is Low, the control circuit 3 inhibits the display of exposure control value and also locks the shutter release to give a warning of drop in voltage of the power source 1.

A₃ is a follower amplifier which is operating only when the output of a gate AND₃ shown in the right-hand edge of FIG. 1 is High. In this position, the information data of exposure factors, film sensitivity and diaphragm aperture value are transmitted to a terminal T₁ of an accessory shoe 8 as described later from the control circuit 3 through the follower amplifier A₃. When the output of gate AND₃ is Low, the follower amplifier A₃ becomes inoperative and the transmission of the exposure factor information data is blocked.

A block 7 enclosed by the dotted line is TTL flash output control circuit to which current is supplied from a power supply transistor Q₆. For purpose of simplification of the drawing, only some chief elements of the circuit elements in the block 7 are shown connected to the transistor Q₆ in FIG. 1. The arrow A in the block 7 suggests that the transistor Q₆ is to be connected to other circuit elements in the block 7. In the TTL flash output control circuit 7 a flashlight measuring photo-detector PD₂ shown as a photo diode is disposed to receive the light reflected upon the film plane. An operational amplifier A₉ and a diode D₂ form a logarithmic compression circuit of the photo current from the photo-detector PD₂. Since the output of film sensitivity is also applied to the logarithmic compression circuit from the follower amplifier A₁, the output from the circuit A₉, D₂ is a composite value resultant from reflected light upon the film plane and film sensitivity. A logarithmic expansion transistor Q₈ has base terminal connected to the output of the logarithmic compression circuit A₉, D₂, emitter terminal connected to the output of a follower amplifier A₈, and collector terminal to an integrating capacitor C₂. The logarithmic expansion transistor Q₈ produces, as its collector current, such current as given by logarithmically expanding the output from the logarithmic compression circuit A₉, D₂. The follower amplifier A₈ generates, as reference voltage, a voltage proportional to the absolute temperature through a temperature compensated constant current source 5 and a resistor R₁₄. Parallel-connected to the integrating capacitor C₂ is a transistor Q₇ which is turned off in response to full opening of the shutter to start charging the capacitor C₂. The turn-Off of this transistor Q₇ is effected by the switch-over of synchro-switch SW₅ from contact a to b in synchronism with the full opening of the shutter.

A₇ is a comparator for forming a TTL flash output control signal. This comparator A₇ compares the integrated output from the integrating capacitor C₂ with the reference voltage from the above described reference voltage source A₄, Q₃, Q₄, R₆ -R₉, D₁. When the integrated output and the reference voltage have just reached a predetermined relationship, the comparator A₇ issues a TTL flash output control signal to terminate flashlight emission. In this manner, the reference voltage source A₄, Q₃, Q₄, R₆ -R₉, D₁ is used as a common reference voltage to the battery checking comparator A₂ and the flash output control signal forming comparator A₇. While the output from the amplifier A₄ has been shown to be directly applied to the comparators A₂ and A₇ as the reference voltage for comparison, it is to be understood that the present invention is not limited to such embodiment shown in FIG. 1 only. As the reference input there may be used also a divided voltage of the output of the amplifier A₄.

For TTL flash output control mode it is generally difficult to attain a proper and correct flash output control under the condition in which the set film sensitivity is extremely high or extremely low. This difficulty is attributable to the limitations imposed on the circuit. To detect whether or not the set film is within the range for proper control there is provided the following detection means:

A pair of comparators A₅ and A₆ constitute a circuit for discriminating whether or not the set film sensitivity is suitable for TTL flash output control. The temperature compensated constant current source 5 and series-connected resistors R₁₂ -R₁₄ constitute a reference voltage source proportional to the absolute temperature. The reference voltage source applies a relatively high reference voltage to one input terminal of the comparator A₅ and a relatively low reference voltage to one input terminal of the other comparator A₆ respectively. The comparator A₅ compares the film sensitivity output of the follower amplifier A₁ with the relatively high reference voltage and produces a High level output when the set film sensitivity is over the upper limit of the above mentioned proper range. The other comparator A₆ makes a comparison between the film sensitivity output of the follower amplifier A₁ and the relatively low reference voltage and generates a High level output when the set film sensitivity is under the lower limit of the proper range. In this manner it is detected by the pair of comparators A₅ and A₆ whether the film sensitivity is within the proper range or not. If not, then the output of gate OR₂ becomes High which serves as a film sensitivity warning signal. The output of gate OR₂ is applied to OR gate OR₁ through AND gate AND₁ and the output of the comparator A₇ is applied to the gate OR₁ through AND gate AND₂. These AND gates AND₁ and AND₂ are alternatively opened. Thus, when the memory switch SW₄ is open, the gate AND₂ is opened and when the switch SW₄ is closed the other gate of AND₂ is opened through inverter INV₁. Transistor Q₅ is controlled by the output of the gate OR₂ when the switch SW₄ is open and controlled by the output of the comparator A₇ when the switch is closed.

The above mentioned accessory shoe 8 of the camera has five terminals T₁ -T₅. The circuit on the camera's side is connected with an electroflash unit attached to the shoe 8 through these terminals T₁ -T₅. Of these five terminals the terminal T₁ is connected to the transistor Q₅ and to the follower amplifier A₃ for transmitting the exposure factor data signal relating to film sensitivity and diaphragm aperture value. The data signal, flash output control signal and film sensitivity warning signal are alternatively transmitted to the electroflash device through the terminal T₁. The second terminal T₂ receives a signal informing of whether the electroflash device is in TTL flash output control mode or not. The terminal T₂ has also another function to transmit a discrimination signal to the electroflash unit. The discrimination signal is a signal informing of whether the camera is in mechanical shutter control mode or in electrical shutter control mode. The third terminal T₃ is connected to the contact b of synchro-switch SW₅ so as to transmit a flash start signal from the camera to the electroflash unit. The fourth terminal T₄ receives from the electroflash unit a signal informing the completion of charging the main capacitor of the flash unit, a flash synchronous time setting signal for changing over the shutter to the flash synchronous time, and other warning signals. The fifth terminal T₅ is a grounded terminal.

Constant current source 6, diodes D₃, D₄ and transistor Q₉ constitute together a TTL output control discriminating circuit for discriminating whether the electroflash unit now mounted on the camera is in TTL flash output control mode or not. The connection point between the constant current source 6 and the diode D₃ is connected to the terminal T₂ to receive TTL output control mode signal from the electroflash unit. The collector of transistor Q₉ is connected to the base of power supply transistor Q₆ through inverter INV₂ so that the transistor Q₆ is turned on to supply current to TTL control circuit 7 only when the TTL flash output control mode discriminating circuit 6, D₃, D₄, Q₉ judges the mode as TTL flash output control mode. A light emitting diode LED₁ is connected, at its anode, to the terminal T₄. This diode LED₁ is disposed within the finder of the camera to serve as a pilot lamp for displaying the completion of charging the main capacitor in the electroflash unit. When the charging of the main capacitor is completed, a charge completion signal in the form of a large current is supplied to the diode LED₁ from the electroflash unit. With this signal, the pilot lamp LED₁ lights on to indicate the completion of charging. Connected in series to the pilot lamp LED₁ is a transistor Q₁₀ which is controlled by the battery checking comparator A₂. When the voltage of camera power source 1 is not sufficiently high, the transistor Q₁₀ is rendered nonconductive to forcedly turn off the pilot lamp LED₁. An interlocking switch SW₆ interlocked with the changeover switch SW₁ is parallel connected to the transistor Q₁₀. The interlocking switch SW₆ is closed when the mechanical shutter control mode is selected by the mode change-over member and is opened when the electrical shutter control mode is selected. Parallel connected to the pilot lamp LED₁ is a switch SW₇ which is turned on only when the shutter time manually set for mechanical shutter control mode is higher in shutter speed than the flash synchronous time. In all other cases, namely in the case where the manually set shutter time is a shutter speed lower than the flash synchronous time or in the case of electrical shutter control mode, the switch SW₇ is opened. A flash synchronous time setting transistor Q₁₁ forcedly sets the shutter to the flash synchronous time when the electrical shutter control mode is selected. The base of the transistor Q₁₁ is connected to the terminal T₄ through resistor R₁₅ and its collector is connected to the control circuit 3. The flash synchronous time setting transistor Q₁₁ is turned on by a flash synchronous time setting signal in the form of a very small current which is supplied to the transistor Q₁₁ from the electroflash unit through terminal T₄ by turn-On, namely supply of the power source of the flash unit. When the transistor Q₁₁ is turned on, the shutter time is forcedly set to the synchronous time controlled by the control circuit 3. At this time, a portion of the very small current flows also into the pilot lamp LED₁. However, this portion of the small current is so preselected as to be too small to turn the pilot lamp on. More particularly, when the transistor Q₁₁ is turned on, the control circuit 3 controls the shutter in the following procedures:

For diaphragm priority automatic exposure control mode, it sets the shutter time to the flash synchronous time.

For shutter priority automatic exposure control mode or programmed automatic exposure control mode, it sets the shutter time to the flash synchronous time and at the same time it inhibits the magnet Mg3 from electrically controlling the diaphragm aperture so as to make it possible to manually set any desired diaphragm value.

For manual exposure control mode, if the manually set shutter time is over the flash synchronous time, then it forcedly changes over the former to the latter. So long as the manually set shutter time is longer than the flash synchronous time, it controls the shutter with the manually set shutter time. In addition, in this case, the control circuit 3 stops the power supply to the circuits associated with automatic exposure control which are unnecessary for manual exposure control, for the purpose of power saving.

Input terminals of AND gate AND₃ are connected to collectors of transistors Q₉ and Q₁₁ through inverters INV₂ and INV₃ respectively. When both of the transistors Q₉ and Q₁₁ are rendered conductive, the output of AND₃ becomes High to operate the follower amplifier A₃ for transmitting exposure factor information signal from the control circuit 3 to the terminal T₁. When the follower amplifier A₃ is inactive, its output impedance becomes infinite.

The manner of operation of the above described circuit provided on the side of camera is as follows:

(1) Operation in the case wherein an electroflash unit of TTL flash output control mode has been mounted on the camera which is in turn in electrical shutter control mode:

In this case, since the camera is set to electrical shutter control mode, the change-over switch SW₁ is closed. By a push-down of the shutter release button up to the first half stroke, the power supply switch SW₂ is closed and the power supply transistor Q₁ is turned on. In the manner later described in detail with reference to FIG. 2, the electroflash unit supplies to the camera's circuit a small current, that is, the flash synchronous time setting signal through the terminal T₄. By this small current the flash synchronous time setting transistor Q₁₁ is turned on so that the control circuit 3 sets the shutter time forcedly to the flash synchronous time. The electronic flash unit of TTL flash output control mode clamps down the potential of terminal T₂ to a sufficiently low level. Detecting this low level potential at the terminal T₂, the TTL flash output control mode discriminating circuit 6, D₃, D₄, Q₉ judges the mode of the electroflash unit to be TTL flash output control mode and renders the output of inverter INV.sub. 2 Low. More concretely, the output of inverter INV₂ is changed to Low by turn-Off of transistor Q₉. With the low level output of inverter INV₂ the power supply transistor Q₆ is turned on to supply power to TTL flash output control circuit 7. In this manner, when there is used an electroflash unit of TTL flash output control mode, the power supply to TTL flash output control circuit 7 on the camera's side is automatically effected. In case of TTL flash output control mode, the electroflash unit need not receive from the camera any exposure factor information signal of film sensitivity and diaphragm aperture. Therefore, the low level output of inverter INV₂ changes the output of gate AND₃ to Low to make inactive the exposure factor information signal transmitting follower amplifier A₃. As soon as the electro flashing unit is ready for flashing, that is to say, as soon as the main capacitor in the flash unit has been charged completely, the flash unit supplies to the camera through the terminal T₄ a relatively large current as a charge completion signal. The output of battery checking comparator A₂ is High and the transistor Q₁₀ is conductive so long as the camera power source 1 is sufficiently high in voltage. Therefore, the charge completion signal turns the pilot lamp LED₁ on because of the switches SW₆ and SW₇ also being open in case of electrical shutter control mode. Naturally, the flash synchronous time setting transistor Q₁₁ remians conductive at this time.

In the above described state wherein the shutter release button has been pushed down only over the first half stroke, the memory switch SW₄ remains opened. Therefore, the gate AND₁ is open and the transistor Q₅ is controlled by the output of the gate OR₂. In this position of the circuit, if the film sensitivity set at the variable resistor R₂ is not suitable for TTL flash output control mode, then either the comparator A₅ or A₆ has a high level output thereby turning the transistor Q₅ on. Turn-On of this transistor Q₅ is transmitted to the electroflash unit through terminal T₁ to bring the previous warning circuit built in the flash unit into operation. The warning circuit makes a pulsating current flow into the circuit in the camera through terminal T₄ to flicker the pilot lamp LED₁. In this manner, if the film sensitivity is not suitable for TTL flash output control mode, there is given a previous warning of it by the flicker of the pilot lamp LED₁ prior to taking a picture. The pulsating current mentioned above has a current value so preset as to turn on the synchronous time setting transistor Q₁₁ even when the pilot lamp LED₁ is turned off. Observing the warning signal, the photographer may give up the intended picture-taking in TTL flash output control mode or may continue it as intended. In the latter case, since the shutter time remains the synchronous time set by the turn-On of transistor Q₁₁ even during warning, a flashlight photographing can be carried out. However, there is no assurance of proper exposure in this case.

When the film sensitivity is within the range suitable for TTL flash output control mode, the outputs of comparators A₅ and A₆ are both Low and transistor Q₅ is nonconductive, namely Off. Therefore, no warning as described above is given.

A further push-down of the shutter release button over the second half stroke closes the release switch SW₃ to actuate the release magnet Mg 1. Thereby a series of mechanical operations such as stopping-down of the diaphragm aperture, upward movement of the quick return mirror, opening of the shutter etc. are started. The memory switch SW₄ is closed immediately before the upward movement of the mirror. In response to the closing of this memory switch SW₄, the gate AND₁ is closed and the gate AND₂ is opened through inverter INV₁. Consequently, after the closing of the memory switch SW₄, the transistor Q₅ is controlled by the output of the flash output control signal forming comparator A₇. Before the shutter is fully opened, the synchro-switch SW₅ is in contact with the contact a and transistor Q₇ is On to short-circuit the integrating capacitor C₂. Therefore, the output of the comparator A₇ is Low and transistor Q₅ is Off. When the shutter is fully opened, the synchro-switch SW₅ is switched over from the contact a to b. On this connection of the synchro-switch SW₅ to the contact b, the electroflash unit is started flashing through terminal T₃. The flashlight reflected upon the film plane is incident on the photo diode PD₂. Also, since the transistor Q₇ is turned off by the connection of the synchro-switch SW₅ to the contact b, the capacitor C₂ starts integration by the collector current of transistor Q₈ corresponding to the film sensitivity and the incident light on the photo diode PD₂. When the integrated voltage reaches the level of the reference voltage of the reference voltage source Q₃, Q₄, A₄, D₁, R₆ -R₉, the signal forming comparator A₇ issues a high level signal, i.e., a flash output control signal. By this signal the transistor Q₅ is turned on to bring into operation a flash terminating circuit in the flash unit through terminal T₁. Thus, flashing is terminated. As will be described in detail, the electroflash unit is so formed as to transmit the signal produced at the terminal T₁ to the previous warning circuit when the synchro-switch SW₅ is in contact with the contact a and transmit the signal to the flash terminating circuit when the switch is in connected to the contact b.

When the shutter is closed the synchro-switch is switched over from contact b to a. Thereby the transistor Q₇ is turned on again to discharge the integrating capacitor C₂. Thereafter, the quick return mirror moves downward and the memory switch SW₄ is opened. From this time, the transistor Q₅ is controlled by the output of the discrimination circuit A₅, A₆, OR₂ for judging whether the film sensitivity is proper or not.

If the voltage of camera power source 1 is not sufficiently high, the output of the battery checking comparator A₂ becomes Low by which the transistor Q₁₀ is turned off to keep the pilot lamp LED₁ unlighted. In this manner, if the camera's circuit is not in the position to normally operate, the pilot lamp always remains to be extinguished to prevent flashlight photographing in TTL flash output control mode under such undesirable condition.

With turn-Off of the power supply switch SW₂ the power source timer circuit Q₁, Q₂, C₁, R₁ starts counting time. A certain predetermined time after the turn-Off of the power supply switch, the power supply transistor Q₁ is turned off to cut off power supply to the whole circuit on camera's side. Consequently, the pilot lamp LED₁ is always extinquished from this time point.

According to the present invention, the output of the discrimination circuit A₅, A₆ and that of the signal forming comparator A₇ are transmitted to the flash unit from the same terminal T₁. To this end, in the shown embodiment, timings of operations on the camera side and on the flash unit side are determined in the following manner:

On the side of camera, the output of the discrimination circuit for judging the suitability of film sensitivity is generated at the terminal T₁ before the actuation (closure) of memory switch SW₄. The output of the comparator A₇ for forming a flash output control signal is generated at the terminal T₁ after the actuation of the switch SW₄. On the side of the electroflash unit, the signal at the terminal T₁ is switched over from previous warning circuit to flash terminating circuit in response to the actuation of synchro-switch SW₅ (connection of the switch SW₅ to the contact b) which is actuated after the actuation of the memory switch SW₅. In this manner, after changing over the signal appearing at the terminal T₁, the circuit of the flash unit to receive the signal is changed over. This embodiment is preferable in view of the fact that it is assured that only the output of the flash output control signal forming comparator A₇ can be applied to the flash terminating circuit of the flash unit without fail. However, other operational timings may be employed to attain the same purpose. For example, timing for changing over the output from that of the discrimination circuit to that of the signal forming comparator may be determined by the actuation of synchro-switch SW₅ in such manner as to make the timing coincident with the timing for switchover from the previous warning circuit to the flash terminating circuit on the side of electroflash unit.

In the embodiment shown in FIG. 1, the suitability of film sensitivity has been detected electrically by use of comparators A₅ and A₆. However, it is to be understood that the method for judging the suitability of film sensitivity is not limited to such electrical discimination method only and there may be used also mechanical means for the same purpose. For example, there may be used a mechanical switch to detect the set film sensitivity. In this case, the electrical output derived from ON-Off of the switch is introduced into the gate AND₁ in substitution for the output from OR gate OR₂ shown in the above embodiment.

(2) Operation in the case wherein an electroflash unit of camera interlocking external flash output control mode has been mounted on the camera which is in turn in electrical shutter control mode:

In this case, since the camera is in electrical shutter control mode, the switches SW₆ and SW₇ are opened. TTL flash output control discrimination circuit 6, D₃, D₄, Q₉ judges that the mode of the flash unit is, in this case, camera interlocking external flash output control mode. After discriminating the mode, the transistor Q₉ becomes conductive, namely On. More concretely, in this case, the electroflash unit does not clamp the potential at terminal T₂ into a lower potential and therefore the transistor Q₉ turned on. By this turn-On of transistor Q₉ the output of inverter INV₂ is changed to High thereby turning the power supply transistor Q₆ off. In this manner, in case of camera interlocking external flash output control mode, no power is supplied to TTL flash output control circuit 7 on camera's side.

With turn-On of the power source of the flash unit, a flash synchronous time setting signal is sent from the flash unit through the terminal T₄. By means of the setting signal, the synchronous time setting transistor Q₁₁ is turned on. As a result, the control circuit 3 forcedly sets the shutter time to flash synchronous time. Also, as a result of turn-On of Q₁₁ the output of inverter INV₃ is changed to High. Since, as previously mentioned, the output of inverter INV₂ is High at the time, the output from the gate AND₃ becomes High. Thereby, the follower amplifier A₃ is brought into operation so that the exposure factor information signal of film sensitivity and diaphragm aperture is transmitted to the flash unit from the control circuit 3 through the amplifier A₃ and terminal T₁.

If the attached electroflash unit has a previous warning function as described above, the information signal transmitted through the terminal T₁ is checked as to whether or not the exposure data is within the range of flash output control ability of the flash unit. If it is judged to be beyond the range, then a pulsating current flows to the camera's circuit to flicker the pilot lamp LED₁ in the same manner as described above in connection with TTL flash output control mode. Flicker of the pilot lamp LED₁ gives the photographer a previous warning to the effect that the combination of set film sensitivity and diaphragm aperture is not suitable for flash output control. Like the case (1), the pulsating current is preset to such value which is sufficient to keep the transistor Q₁₁ conductive even when the pilot lamp LED₁ is turned off.

By closing the release switch SW₃ the shutter is fully opened in the same manner as above and the synchro-switch SW₅ is switched over from contact a to b to make the flash unit start flashing. At the same time, integration of the quantity of reflected flashlight is started using a photo receptor element provided in the flash unit. The electroflash unit terminates flashing after a certain predetermined time according to the integrated light quantity and the exposure factor information signal of film sensitivity and diaphragm aperture introduced from the camera's side. If the power supply transistor Q₁ is left nonconductive or if the voltage of the camera power source 1 is too low, then the pilot lamp LED₁ remains always to be extinguished as described above.

In this manner, when the attached electroflash unit is in camera interlocking external flash output control mode, no power is supplied to TTL flash output control circuit. Information of exposure factors, film sensitivity and diaphragm is automatically introduced into the flash unit.

(3) Operation in the case wherein an electroflash unit of independent external flash output mode has been mounted on the camera which is in turn in electrical shutter control mode:

In this case, the flash unit does not use any exposure factor signal information of film sensitivity and diaphragm aperture from the camera. Instead, the flash unit carries out an independent flash output control operation.

(4) Operation in the case wherein the camera is in mechanical shutter control mode:

When the mechanical shutter control mode is selected, the change-over switch SW₁ is opened and therefore there takes place no power supply at all to the side of camera. When the shutter time manually set is longer than the flash synchronous time and the attached electroflash unit is of TTL flash output control mode or camera interlocking external flash output control mode, then the following operations take place:

Since the shutter time is longer than the flash synchronous time, the switch SW₇ is open in this case. Also, since no power is supplied to the camera's circuit from the power source 1, TTL flash output control circuit 7 is inactive. Therefore TTL flash output control is impossible. The control circuit 3 and the amplifier A₃ are also inactive. An information signal of film sensitivity and diaphragm aperture can not be transmitted to the flash unit from the camera. Therefore, camera interlocking external flash output control is impossible also. The flash unit in TTL flash output control mode will detect the absence of voltage at the terminal T₂. The flash unit in camera interlocking external flash output mode will detect the absence of voltage at the terminal T₁. In either case, the flash unit brings its own warning circuit into operation in response to the detection. Thereby a pulsating current is supplied to the terminal T₄. As mechanical shutter control mode is selected on the camera's side, the switch SW₆ is now in its closed position. Therefore, the pulsating current from the flash unit's side is allowed to flow through the pilot lamp LED₁ and the switch SW₆. Thus, the pilot lamp LED₁ flickers, which indicates that neither TTL flash output control nor camera interlocking flash output control is possible.

On the contrary, when an electroflash unit of independent external flash output control mode is mounted on the camera, a charge completion signal is supplied to the camera's side from the flash unit through terminal T₄ after completing the charging of the main capacitor in the flash unit. This signal flows through the pilot lamp LED₁ and switch SW₆ from the terminal T₄. Therefore, the pilot lamp LED₁ lights up to indicate that the flash unit is ready for flashing.

On the other hand, the camera is provided with a mechanical release mechanism which can be operated by pushing down the shutter release button because mechanical shutter control mode has been selected in this case. With the operation of the mechanical release mechanism, a series of operations such as stopping down the diaphragm, driving the quick return mirror and opening the shutter are performed successively in the same manner as that at the actuation of the release magnet Mgl described above. When the shutter is fully opened, the synchro-switch SW₅ is connected to the contact b to cause flashing of the flash unit. The flashlight reflected by the object is received by a photo-detector in the flash unit to control the flashlight emission.

As readily understood from the above, when the mechanical shutter controls mode is selected, the mode which can be used for flash is limited to the independent external flash output control mode only. If the mechanical controlled shutter time then selected is shorter the flash synchronous time, then switch SW₇ is closed to short-circuit the pilot lamp LED₁. Therefore, in this case, the pilot lamp remains extinguished which prevents an erroneous picture-taking with flashlight.

(5) Operation in the case wherein no electroflash unit has been mounted yet on the camera or the power source of the electroflash unit has not been supplied even after mounting it on the camera which is in turn in electrical shutter control mode:

In this case, the transistor Q₉ is conductive and the synchronous time setting transistor Q₁₁ is conductive because the electroflash unit has not been mounted on the camera yet or, even if mounted, its power source is not supplied. Conductivity of the transistor Q₉ renders the power supply transistor Q₆ nonconductive, namely Off, to inhibit power supply to TTL flash output control circuit 7 for purpose of power saving. Also, since the transistor Q₁₁ is nonconductive, the control circuit 3 does not carry out any forced setting of the shutter to the flash synchronous time. The shutter is controlled depending on the measured brightness of the object or on the shutter time manually set at that time. The output of AND gate AND₃ is Low because of transistor Q₁₁ being nonconductive. The low level output of the gate AND₃ makes the amplifier A₃ inactive. Consequently, in this case, there is produced no information signal of film sensitivity and diaphragm aperture at the terminal T₁.

Hereinafter, a description will be given of an electroflash unit in accordance with the invention used together with the above described camera.

FIG. 2 shows an embodiment of electroflash unit according to the present invention. The electroflash unit is so designed as to be operable in any mode selection from the group consisting of TTL flash output control mode, independent external flash output control mode and non-flash output control mode.

The electroflash unit shown in FIG. 2 is provided with a mounting foot 10 having five terminals T'₁ -T'₅. To mount the flash unit to a camera, the mounting foot 10 is engaged in the above mentioned accessory shoe 8 of the camera. In engagement, the five terminals T'₁ -T'₅ on the flash unit are connected to the corresponding five terminals T₁ -T₅ on the camera. Designated by 15 at the right-hand edge of FIG. 2 is a power source for flash unit. Connected in series to the flash unit power source 15 is a power source switch SW₁₂ having three contacts f, g and h. When the contact f is selected by the power source switch SW₁₂, the power supply to the flash unit is carried out interlocking with the condition of power supply to the camera. This mode is referred to as camera interlocking power source mode. When the contact g is selected, the power supply is not carried out. When the contact h is selected, the power supply to the flash unit is carried out independently of the camera.

Transistors Q₃₃ -Q₃₅, resistors R₃₁, R₃₂ and capacitor C₁₇ constitute a power supply control circuit. When the camera interlocking power source mode contact f is selected by the power source switch SW₁₂, the power supply control circuit detects, through terminals T₂ and T'₂, whether or not power is being supplied to the camera. If the camera's circuit is in the state of power supply, then the power supply control circuit Q₃₃ -Q₃₅, R₃₁, R₃₂ automatically brings the flash unit into the state of power supply. On the contrary, if the camera is in the state of non-power supply, then the control circuit prevents power supply to the flash unit.

14 is a booster circuit for boosting the voltage of the power source 15. A main capacitor C₁₆ is charged with the boosted source voltage through diode D₅₀. Zener diode ZD₂ and resistors R₂₉ and R₃₀ consitute together a charge completion detecting circuit. When the charge on the main capacitor C₁₆ reaches a predetermined level for flashing, the detection circuit detects it. 13 is a flash discharge tube which is flashed by the discharge current from the main capacitor C₁₆. 12 is a trigger circuit which causes the discharge tube 13 to start flashing in response to the actuation of the synchro-switch on the camera. The trigger circuit 12 has also another function to terminate the flashlight emission in response to the actuation of a flash terminating circuit 11 which is actuated by means of a flash terminating signal, i.e., flash output control signal. LED₁₃ is a pilot lamp formed of a light emitting diode, and is controlled by transistor Q₃₁ connected in series to the pilot lamp. R₃₃ is a current limiting resistor whose one end is connected to the positive pole of the power source 15 through power source switch SW₁₂. The other end of the resistor R₃₃ is connected to the terminal T'₄. When power is supplied to the flash unit, the current limiting resistor R₃₃ supplies a flash synchronous time setting signal, that is, a small current to the transistor Q₁₁ of the camera shown in FIG. 1 through terminal T'₄, T₄. Transistor Q₃₂ and resistor R₃₄ are connected in series with each other and connected in parallel to the current limiting resistor R₃₃. The transistor Q₃₂ is controlled by the output of the charge completion detecting circuit ZD₂, R₂₉, R₃₀ through gates NAND₁₁ and AND₁₅. The transistor Q₃₂ is nonconductive before completing the charging of main capacitor C₁₆ and it is rendered conductive after completing the charging. When the transistor Q₃₂ is turned on, resistors R₃₃ and R₃₄ are connected in parallel to each other. Therefore, a large current serving as a charge completion signal is supplied to the circuit on the camera through terminal T'₄, T₄ to light on the pilot lamp LED₁ on the camera. Naturally, this large current maintains conductivity of the transistor Q₁₁ on the camera.

Resistors R₂₅ -R₂₈, comparator A₁₃, capacitor C₁₅ and transistor Q₃₀ form a warning oscillating circuit which operates when the transistor Q₃₀ is rendered nonconductive. In operation, the warning circuit generates an oscillation output of several HZ at the output terminal of comparator A₁₃. The oscillation output is applied to the transistor Q₃₁ through gates AND₁₆ and OR₁₄ to control ON-OFF of the transistor Q₃₁ with the above mentioned oscillation frequency. Thereby the pilot lamp LED₁₃ on the side of flash unit is flickered on one hand. On the other hand, the oscillation output is applied to the transistor Q₃₂ through gate AND₁₅ to control ON-OFF of Q₃₂ with the above oscillation frequency thereby flickering the pilot lamp LED₁ on the side of camera.

A₁₂ is a comparator for detecting high speed shutter time mechanically controlled. The comparator A₁₂ compares the voltage at terminal T'₄ with a reference voltage formed by voltage dividing resistors R₂₂ and R₂₃ to detect that the shutter time set for mechanical shutter control mode is a higher speed shutter time namely shorter than the flash synchronous time. More concretely, in case of electrical shutter control mode, the potential at terminal T'₄ is the sum of base-emitter voltage of transistor Q₁₁ and voltage of resistor R₁₅ as seen in FIG. 1. In case of mechanical shutter control mode and high speed shutter time, the potential at the terminal T'₄ is ground potential because of switches SW₆ and SW₇ being closed. As the reference voltage mentioned above, a value between the above sum and the ground potential is selected. By setting the reference voltage to such level it is assured that the output of comparator A₁₂ becomes High only when the shutter time set in mechanical shutter control mode is a higher speed shutter time than the synchronous shutter time and remains Low for all other times. When the comparator A₁₂ detects the high speed shutter time and therefore its output becomes High, it closes the gate of AND₁₇ through an inverter INV₁₄ to disconnect transistor Q₃₁ from the output of the charge completion detecting circuit ZD₂, R₂₉, R₃₀ and also to turn transistor Q₃₀ off through gate NOR₁₀ to thereby bring the warning oscillation circuit R₂₅ -R₂₈, A₁₃, C₁₅, Q₃₀ into operation.

Transistors Q₂₈ and Q₂₉, comparator A₁₁, capacitor C₁₄ and resistor R₂₁ constitute together a circuit for detecting that flash output control has been unexecuted. When there is produced no flash output control signal during the longest flash time of the attached electroflash unit after start of flashing, the unexecuted flash output control detecting circuit detects it and the output of the comparator A₁₁ becomes High for a certain determined time length. By this high level output, that is, an unexecuted flash output control detection signal, AND gate AND₁₆ is opened through gate OR₁₃ and also transistor Q₃₀ is turned off through gate NOR₁₀. Therefore, the warning oscillation circuit starts operating. At the same time, the operation of booster is inhibited or suppressed. In this case, the main capacitor C₁₆ is completely discharged because no flash output control is executed. Therefore, the output of the charge completion detecting circuit ZD₂, R₂₉, R₃₀ becomes Low by which the gate AND₁₇ is closed and the gate AND₁₅ is opened through gate NAND₁₁. Consequently, when no flash output control was executed, both of the pilot lamps LED₁ and LED₁₃ continue flickering for a certain predetermined time after the end of flashing.

The divided voltage formed by voltage dividing resistors R₂₂ and R₂₃ mentioned above is used as a common reference voltage to the comparators A₁₁ and A₁₂.

Within the electroflash unit a photo diode PD₁₀ is disposed to receive the flashlight reflected by the object. When transistor Q₂₇ is turned off, an integrating capacitor C₁₂ starts to integrate the photo current. An integrating capacitor C₁₃ is connected in parallel to the integrating capacitor C₁₂ when switch SW₁₁ is closed. The switch SW₁₁ is opened when the diaphragm of the camera is set at a predetermined value for example f4. It is closed when the diaphragm is stopped down to a predetermined larger diaphragm value for example f8. Therefore, when the predetermined larger diaphragm value is selected, the integration time required for the terminal voltage of condensers C₁₂, C₁₃ to reach a determined level increases. These elements, photo diode PD₁₀, capacitors C₁₂, C₁₃, transistor Q₂₇ and switch SW₁₁ constitute a light quantity integrating circuit for independent flash output control mode. In case of the independent flash output control mode, the integrated output of the light quantity integrating circuit PD₁₀, C₁₂, C₁₃, Q.sub. 27, SW₁₁ and a reference voltage formed by Zener diode ZD₁ are compared each other by a comparator A₁₀. When they are in a predetermined relationship to each other, the comparator A₁₀ generates a flash output control signal by means of which flashlight emission is terminated. This flash output control signal is applied to a flash terminating circuit 11 through gates AND₁₃, OR₁₁ and AND₁₂.

Capacitor C₁₀ and resistors R₆₀ and R₆₁ constitute a differentiation circuit which is connected to the terminal T'₃. When the synchro-switch SW₅ of camera is connected to the contact b, the differentiation circuit generates a differentiation pulse. Transistors Q₂₄, Q₂₅, resistor R₂₀ and capacitor C₁₁ constitute a monostable multivibrator circuit which is triggered by the differentiation pulse generated from the differentiation circuit C₁₀ R₆₀, R₆₁. When triggered, the monostable circuit produces a high level output at the collector of transistor Q₂₄ for a certain time as long as or longer than the maximum flashing time of the electroflash unit. The high level output brings a trigger circuit 12 into operation to start flashing. Also, it turns the transistors Q₂₆ and Q₂₇ off to start the above mentioned integration of light quantity. Furthermore, the high level output is applied to gates NAND₁₀, AND₁₁ and AND₁₂. The gate AND₁₁ forms together with OR gate OR₁₀ a latch circuit. The function of this latch circuit AND₁₁, OR₁₀ is to latch the flash output control signal until the decay of output of the monostable circuit. More particularly, when a flash output control signal is applied to the gate OR₁₀ from the gate AND₁₂ during the output generation from the monostable circuit, the latch circuit AND₁₁, OR₁₀ latches the signal until the output of the monostable circuit becomes extinct even if the signal has already disappeared. The output of this latch circuit, that is, the output from the gate OR₁₀ is applied to the gate NAND₁₀ through inverter INV₁₃ on one hand and also used to control transistor Q₂₉ on another hand.

Q₂₃ is a transistor whose base is connected to the terminal T'₃. The transistor Q₂₃ is rendered conductive when the synchro-switch SW₅ on the camera's side is in contact with the contact b. The output of inverter INV₁₂ is turned to High by conductivity of this transistor Q₂₃ to close the gate AND₁₄. When the synchro-switch SW₅ is in connection with the contact a, the transistor Q₂₃ is rendered nonconductive by which the gate AND₁₄ is opened through inverter INV₁₂. The function of this transistor Q₂₃ is as follows:

In case of TTL flash output control mode, a signal informing of the suitability of set film sensitivity is transmitted from camera's side to the electroflash unit through terminal T₁, T'₁ before operating the shutter release. After the shutter release operation, TTL flash output control signal is transmitted to the flash unit from the camera through terminal T₁, T'₁. The film sensitivity suitability signal has to be introduced into the warning oscillation circuit R₂₅ -R₂₈, A₁₃, C₁₅, Q₃₀. The TTL flash output control signal has to be introduced into the flash terminating circuit 11. This is attained by transistor Q₂₃. Before shutter releasing operation, more concretely, during the time of the synchro-switch SW₅ being in connection with the contact a, the transistor Q₂₃ opens the gate AND₁₄ so as to allow the film sensitivity suitability signal to enter the warning oscillation circuit through the gate AND₁₄. After the shutter releasing operation, the transistor Q₂₃ closes the gate AND₁₄ to prevent any input of flash output control signal into the warning oscillation circuit.

The state of transistor Q₅ on camera's side is detected by detecting the potential at terminal T'₁ by transistor Q₂₂. When the transistor Q₅ is rendered nonconductive which means that the film sensitivity set in TTL flash output control mode is proper or that no TTL flash output control signal is being generated, the transistor Q₂₂ is rendered nonconductive to change the output of gage AND₁₀ to Low. On the contrary, when the set film sensitivity is not proper, that is to say, when the transistor Q₅ is rendered conductive, the transistor Q₂₂ is also rendered conductive so as to bring the warning oscillation circuit into operation through gates AND₁₀, OR₁₂, AND₁₄ and NOR₁₀. Also, when the transistor Q₅ is rendered conductive as a result of generation of TTL flash output control signal, the transistor Q₂₂ is rendered conductive to apply a signal to the flash terminating circuit 11 through gates AND₁₀, OR₁₁ and AND₁₂.

The mode selection switch SW₁₀ is externally operated to select any one of contact c for TTL flash output control mode, contact d for independent flash output control mode and contact e for non-flash output control. The contact e is connected to a display diode LED₁₂ and a NAND gate NAND₁₀. Therefore, when the contact e is selected by the mode selection switch SW₁₀ , the light emitting diode LED₁₂ lights up to indicate it. At the same time, a low level signal is applied to the gate NAND₁₀ to keep transistor Q₂₈ nonconductive. Thereby the unexecuted flash output control detection Q₂₈, Q₂₉, A₁₁, C₁₄, R₂₁ are made inactive.

The contact d is connected to an independent flash output control mode display diode LED₁₁ and also to an AND gate AND₁₃ through inverter INV₁₁. Therefore, when this contact d is selected, the display diode LED₁₁ lights up to indicate the selected mode, independent flash output control mode. At the same time, the gate AND₁₃ is opened to allow a flash output control signal to be transmitted to the flash terminating circuit 11 from the flash output control signal forming comparator A₁₀.

The contact c is connected to TTL flash output control mode display diode LED₁₀. It is also connected, through inverter INV₁₀, to transistor Q₂₁, gate AND₁₀ and gate AND₁₄. Therefore, when this contact c is selected, the display diode LED₁₀ is turned on to indicate the selected mode, TTL flash output control mode and also transistor Q₂₁ is rendered conductive. At the same time, the gates AND₁₄ and AND₁₀ are opened. Film sensitivity suitability signal in TTL flash output control mode and TTL flash output control signal are allowed to be transmitted through transistor Q₂₂.

Transistors Q₂₀ and Q₂₁ and diode D₁₀ constitute a circuit for generating a TTL flash output control mode selection signal. When the contact c is selected, both of the transistors Q₂₀ and Q₂₁ in this circuit is rendered conductive to or hold the potential at terminal T'₂ into a low level. Concretely, the low level is a value as given by adding the forward voltage of diode D₁₀ to the base-emitter voltage of transistor Q₂₀. This low level potential constitutes the TTL flash output control mode selection signal. By this mode selection signal the transistor Q₉ on camera's side is rendered nonconductive.

The manner of operation of the above described electroflash unit is as follows:

(IA) Operation in the case wherein TTL flash output control mode is selected by the flash unit the power source of which is in camera non-interlocking mode, and the mode on camera's side is electrical shutter control mode:

In this case, on the side of the electroflash unit shown in FIG. 2, its mode selection switch SW₁₀ is in connection with the contact c and its power source switch SW₁₂ is in connection with the contact h. On the side of the camera shown in FIG. 1, its change-over switch SW₁ is closed and switches SW₆ and SW₇ are open. As the power source of the flash unit is supplied, a small current determined by the current limiting resistor R₃₃ flows into the camera's side through terminal T'₄, T₄ as a flash synchronous time setting signal. This signal renders the flash synchronous time setting transistor Q₁₁ conductive to forcedly set the shutter to the flash synchronous time. The small current keeps the pilot lamp LED₁ on camera's side in the state of extinction essentially.

With the selection of contact c by the mode selection switch SW₁₀, the transistors Q₂₀ and Q₂₁ of TTL flash output control mode selection signal generator Q₂₀, Q₂₁, D₁₀ is rendered conductive to clamp the potential at terminal T'₂, T₂ to a lower level. Thus, TTL flash output control mode selection signal is supplied to the circuit on the camera to render its transistor Q₉ nonconductive. By nonconductivity of this transistor, on the side of camera, transistor Q₆ is rendered conductive and therefore power is supplied to TTL flash output control circuit 7.

In this manner, when TTL flash output control mode is selected, the flash unit issues TTL flash control mode selection signal which is detected by TTL flash output control mode discriminating circuit 6, D₃, D₄, Q₉ on camera's side to automatically start the power supply to TTL flash output control circuit 7. In this connection it should be noted that if the electroflash unit receives no power from its power source 15, the selection signal generator Q₂₀, Q₂₁, D₁₀ does not issue the above mentioned selection signal even when TTL flash output control mode is selected. Therefore, in this case, power is not supplied to TTL flash output control circuit 7 on camera's side. A further effective power saving of the camera's battery 1 can be attained by it.

On the other hand, since the camera is in electrical shutter control mode, the mechanically controlled high speed shutter time detecting comparator A₁₂ in the electroflash unit generates low level output which opens the gate AND₁₇ through inverter INV₁₄. Therefore, when the main capacitor C₁₆ has completely been charged up to a preselected value, the charge completion detecting circuit ZD₂, R₂₉, R₃₀ applies a High output to the gate AND₁₇ to render transistor Q₃₁ conductive. This turns the pilot lamp LED₁₃ on to indicate the completion of charging. Also, when the film sensitivity set on the camera is proper with respect to TTL flash output control mode, the transistor Q₅ is rendered nonconductive, which in turn renders the transistor Q₂₂ on the flash unit also nonconductive through terminal T₁, T'₁. Thereby the output of the gate AND₁₀ is changed to Low. Low output of the gate AND₁₀ and conductivity of transistor Q₂₀ make the output of the gate OR₁₂ Low to cause the gate AND₁₄ to be Low. Since the unexecuted flash output control detecting circuit Q₂₈, Q₂₉, A₁₁, C₁₄, R₂₁ remains still inactive, the output of its comparator A₁₁ is also Low. As mentioned above, the output of comparator A₁₂ also is Low. As all the outputs of comparators A₁₁ and A₁₂ and gate AND₁₄ are Low, the output of the gate NOR₁₀ becomes High to render transistor Q₃₀ conductive. In this state, the warning oscillation circuit R₂₅ -R₂₈, A₁₃, C₁₅ is inactive and the output of its comparator A₁₃ remains unchanged at High. In this manner, when the set film sensitivity is proper, there is performed no warning operation as a matter of course.

On the other hand, the output of NAND gate NAND₁₁ is changed to Low by High outputs of gate NOR₁₀ and the charge completion detecting circuit R₂₉, R₃₀, ZD₂. By this Low output of the gate NAND₁₁ the transistor Q₃₂ is rendered conductive through the gate AND₁₅. On conductivity of this transistor Q₃₂, resistors R₃₃ and R₃₄ are connected in parallel with each other. Consequently, a relatively large current serving as a charge completion signal flows into the pilot lamp LED₁ on the camera through terminal T'₄, T₄ to lighten the pilot lamp. In this manner, when the charging of the main capacitor C₁₆ is completed, both of the pilot lamp LED₁ on the camera and the pilot lamp LED₁₃ on the flash unit are turned on.

If the set film sensitivity is not proper for TTL flash output control mode, then the transistor Q₅ on camera's side becomes conductive. Therefore, the transistor Q₂₂ on the flash unit also becomes conductive. The output of AND₁₀ is changed to High by conductivity of Q₂₂ and then the output of OR₁₂ is changed to High. On the other hand, since the shutter has not been released yet and the synchro-switch SW₅ is in connection with the contact a, the transistor Q₂₃ is rendered nonconductive and therefore the output of inverter INV₁₂ is High. As all the outputs of OR₁₂, INV₁₀ and INV₁₂ are High, the output of AND₁₄ becomes High by which the transistor Q₃₀ is rendered nonconductive through gate NOR₁₀. Thereby the warning oscillation circuit R₂₅ -R₂₈, A₁₃, C₁₅ is brought into operation. Thus, the oscillation circuit generates an oscillation output of several Hz at the output terminal of comparator A₁₃. This oscillation output controls the transistor Q₃₂ through gate AND₁₅ to turn the transistor on and off at a frequency of several Hz. Therefore, the pilot lamp LED₁ on the camera flickers as a warning of the fact that the set film sensitivity is not proper. At the time, both of the comparators A₁₁ and A₁₂ are Low in output level, which closes the gate AND₁₆ through OR₁₃. Therefore, the oscillation output of the warning circuit can not be transmitted to the transistor Q₃₁. Thus, the transistor Q₃₁ is controlled solely by the output of the charge completion detecting circuit ZD₂, R₂₉, R₃₀. Consequently, the pilot lamp LED₁₃ on the flash unit does not flicker at this time.

When the shutter has been opened fully, the synchro-switch SW₅ is connected to the contact b and the monostable circuit Q₂₄, Q₂₅, R₂₀, C₁₁ on the flash unit is triggered by the differentiation circuit C₁₀, R₆₀, R₆₁. After triggered, the transistor Q₂₄ of the monostable circuit Q₂₄, Q₂₅, R₂₀, C₁₁, continue to be conductive for a certain determined time longer than the maximum flash time of the flash unit. In response to conductivity of this transistor Q₂₄, the trigger circuit 12 makes the flash discharge tube 13 start flashing and at the same time it brings the latch circuit AND₁₁, OR₁₀ into the waiting position which is the same position as the reset position. Conductivity of the transistor Q₂₄ further opens the gate AND₁₂, inverses the output of NAND₁₀ to Low to render transistor Q₂₈ conductive. As the transistor Q₂₉ is nonconductive, conductivity of the transistor Q₂₈ causes the condenser C₁₄ to be fully charged in a moment. Also, transistor Q₂₃ is rendered conductive by the connection of the synchro-switch SW₅ to the contact b. Conductivity of this transistor Q₂₃ closes the gate AND₁₄ through inverter INV₁₂ to block the transmission of signals from the camera to the flash unit through terminal T'₁ thereafter.

Thereafter, TTL flash output control circuit 7 carries out the integration of light quantity. When the integrated light quantity reaches a predetermined value, the circuit 7 issues a flash terminating signal, that is, a flash output control signal by which the transistor Q₅ on the camera's side is rendered conductive. Conductivity of this transistor Q₅ renders the transistor Q₂₂ conductive to change the output of the gate AND₁₀ to High. By this High output of AND₁₀ the output of AND₁₂ is changed to High through OR₁₁ to bring the flash terminating circuit 11 into operation. Thus, flashing of the flash discharge tube 13 is terminated. At the same time, the output of AND₁₂ triggers the latch circuit AND₁₁, OR₁₀. Thereby the outputs of the gates AND₁₁ and OR₁₀ are held at High. Even if the output of AND₁₂ is inverted to Low after that, the holding of High level continues so long as the output of the monostable circuit Q₂₄, Q₂₅, R₂₀, C₁₁ is present. High output of the gate OR₁₀ of this latch circuit inverses the output of the gate NAND₁₀ to High, renders transistor Q₂₈ nonconductive and also renders transistor Q.sub. 29 conductive. Therefore, the capacitor C₁₄ is discharged in a moment.

In this manner, the capacitor C₁₄ of the unexecuted flash output control detecting circuit Q₂₈, Q₂₉, A₁₁, C₁₄, R₂₁ is fully charged in a moment simultaneously with the start of flashing and its comparator A₁₁ is reversed to High so as to operate the warning oscillation circuit through NOR₁₀ and Q₃₀. However, when a flash output control operation has once been executed, the capacitor C₁₄ is discharged by the flash output control signal then generated. This discharging of the capacitor C₁₄ takes place immediately after the start of flashing (within the time of several milliseconds at the latest). By this instant discharge, the comparator A₁₁ is again returned back to Low output to stop the operation of the warning oscillation circuit. Therefore, in case that the flash output control has been executed, the unexecuted flash output control detecting circuit does not actuate the warning oscillation circuit virtually and there is given no warning of unexecuted flash output control.

There is a case wherein the integrated light quantity in TTL flash output control circuit 7 of the camera can not reach the predetermined value even after the electroflash unit has emitted the maximum amount of flashlight. In such case, there is generated no flash output control signal. Therefore, the latch circuit AND₁₁, OR₁₀ is not triggered and the output of its OR gate OR₁₀ remains unchanged at Low. Transistor Q₂₉ also remains nonconductive. As a result, the charge on the capacitor C₁₄ is discharged not instantly but gradually through resistor R₂₁. The comparator A₁₁ holds its High output for a certain time long determined by the time constant of resistor R₂₁ and capacitor C₁₄, for example, several seconds. During this holding time, it brings the warning oscillation circuit R₂₅ -R₂₈, A₁₃, C₁₅ into operation. This high level output of the comparator A₁₁ opens the gate AND₁₆ at the same time. On the other hand, since the main capacitor C₁₆ has been already discharged completely, the output of the charge completion detecting circuit ZD₂, R₂₉, R₃₀ at the time is Low which opens the gate AND₁₅ through NAND₁₁. Consequently, the oscillating output of the warning oscillation circuit controls the transistor Q₃₂ through AND₁₅ and also controls the transistor Q₃₁ through AND₁₆ and OR₁₄. Thus, the pilot lamp LED₁ on the camera as well as the pilot lamp LED₁₃ on the flash unit begin flickering.

In this manner, if the flash output control has not been executed, the unexecuted flash output control detecting circuit detects the absence of a flash output control signal and flickers both of the pilot lamps for a determined time to indicate the unexecution of flash output control. The high level output of the comparator A₁₁ indicative of the unexecution of flash output control, not only operates the warning oscillation circuit in the above described manner but also inhibits or suppresses the operation of booster circuit 14 at the same time. This inhibition or suppression has an effect to prevent voltage drop of the power source 15 and assure the supply of sufficient power to various circuits for giving a warning of the unexecuted flash output control.

As readily understood from the foregoing, in the case wherein the electronic flash unit is in TTL flash output control mode and the camera is in electrical shutter control mode, two different kinds of time series signals are transmitted to the flash unit from the camera through the same terminal T₁, T'₁. More particularly, before shutter releasing operation there is transmitted to the flash unit a signal informing of the suitability of set film sensitivity and after shutter releasing operation there is transmitted TTL flash output control signal. The electronic flash unit directs the different signals to different circuits in response to a synchronizing signal transmitted thereto from the camera through another terminal T₃, T'₃. More particularly, before receiving the synchronizing signal, the flash unit supplies the above suitability signal to the warning oscillation circuit. After receiving the synchronizing signal, the flash unit supplies the above flash output control signal to the flash terminating circuit etc. In this manner, even using the same terminal for transmitting two different kinds of signals, the apparatus can deliver the respective signals to the respective determined circuits therefor.

(IIA) Operation in the case wherein the electronic flash unit is in the same mode as in the above (IA), namely TTL flash output control mode with its power source being in a camera non-interlocking mode but the camera is in mechanical shutter control mode with its shutter time being longer than the flash synchronous time:

In this case, no power is supplied to the circuit on the camera because of its change-over switch SW₁ being open. Therefore, the TTL flash output control circuit 7 on the camera is inactive. In this case, even if TTL flash output control mode is selected in the flash unit, the selected TTL flash output control is impossible to carry out. To indicate it, the apparatus previously gives a warning to the effect that TTL flash output control is impossible.

Since no power is supplied to the camera's circuit, the current source 6 of the camera is inactive and there is no current supply from the camera to the flash unit through terminal T₂, T'₂. Therefore, on the side of flash unit, transistor Q₂₀ is nonconductive and the output of gate OR₁₂ is High. Also, as the terminal c is selected, the output of inverter INV₁₀ becomes High. Before releasing the shutter, transistor Q₂₃ is nonconductive and therefore the output of inverter INV₁₂ is also High. All the outputs of OR₁₂, INV₁₀ and INV₁₂ are now High, which makes the output of gate AND₁₄ High and the output of NOR₁₀ Low. Consequently, transistor Q₃₀ is rendered nonconductive to actuate the warning oscillation circuit R₂₅ -R₂₈, A₁₃, C₁₅. The low level output of NOR₁₀ opens the gate AND₁₅ through NAND₁₁. Therefore, the oscillating output of the warning oscillation circuit controls the transistor Q₃₂ through gate AND.sub. 15. Thereby, a pulsating current is supplied to the camera through terminal T'₄. Since switch SW₆ is closed although transistor Q₁₀ is nonconductive, the pulsating current is allowed to flow into the pilot lamp LED₁ on the camera. Thus, the pilot lamp LED₁ flickers as a warning of impossibility of TTL flash output control. But, the pilot lamp LED₁₃ on the side of flash unit does not flicker at the time. The reason for this is as follows:

In the case now being discussed, the shutter time is longer the flash synchronous time and therefore the output level of the mechanically controlled high speed shutter time detecting comparator A₁₂ is Low. On the other hand, the comparator A₁₁ in the unexecuted flash output control detecting circuit is also Low in output level. Therefore, the output of gate OR₁₃ becomes Low which closes the gate AND₁₆. Consequently, the transistor Q₃₁ becomes independent of the oscillating output of the warning oscillation circuit. The transistor Q₃₁ is controlled solely by the output of the charge detection circuit. Therefore, the pilot lamp LED₁₃ on the electroflash unit does not flicker at this time. Warning of impossibility of TTL flash output control is given only by the pilot lamp LED₁ on the camera's side. The pilot lamp LED₁₃ on the electroflash unit is not used for this purpose for the following reason:

So long as the power supply transistor Q₁ is nonconductive, no power is supplied to the circuit on the camera even when the camera is in electrical shutter control mode and the change-over switch SW₁ is closed. The electronic flash unit mistakes this state for the opened state of the change-over switch SW₁. If the pilot lamp LED₁₃ on the flash unit is so formed as to flicker for warning whenever no power is supplied to the camera's circuit, then the pilot lamp LED₁₃ will flicker also in the case of no power supply to the camera's side caused by nonconductivity of transistor Q₁. To prevent it, in the shown embodiment, the pilot lamp LED₁₃ of the flash unit is so formed as not to flicker for the above warning. The pulsating current supplied to the camera from the flash unit through terminal T₄ at that time does not cause any erroneous warning display because when the camera's power supply transistor Q₁ is nonconductive, transistor Q₁₀ is rendered nonconductive to forcedly turn, the pilot lamp LED₁ off.

In the above, description has been made referring to the case wherein the camera is provided with TTL flash output control circuit 7 but there is no power supply to it. However, it is to be understood that the above description may be applied also to such camera without TTL flash output control circuit. In this case, the camera is not provided with terminal T₂. Therefore, the pilot lamp LED₁ on the camera flickers to give a warning that the selection of TTL flash output control mode at the flash unit is improper.

As will be understood from the foregoing, the transistor Q₂₀ in the above apparatus is not only a member of TTL flash output control mode selection signal generating circuit but also constitutes a detection circuit for detecting whether or not TTL flash output control is possible on the camera's side. When the camera is in the state of non-TTL flash output control, the transistor Q₂₀ is rendered nonconductive to actuate the warning oscillation circuit R₂₅ -R₂₈, A₁₃, C₁₅ as described above.

(IIIA) Operation in the case wherein the electroflash unit is in independent flash output control mode with its power source switch being in camera non-interlocking mode and the camera is in electrical shutter control mode or mechanical shutter control mode with the flash synchronous time:

In this case, since the mode selection switch SW₁₀ of the flash unit is in connection with the contact d, the output of inverter INV₁₁ is High by which the gate AND₁₃ is opened. In this state, the output of the independent flash output control signal forming comparator A₁₀ can actuate the flash terminating circuit 11 and the unexecuted flash output control detecting circuit Q₂₈, Q₂₉, A₁₁, C₁₄, R₂₁ through the gates AND₁₃, OR₁₁ and AND₁₂. When the shutter is fully opened and the synchro-switch SW₅ is connected to the contact b, the differentiation circuit C₁₀, R₆₀, R₆₁ renders the transistor Q₂₄ of the monostable circuit conductive by its differentiation pulse in response to the synchro-switch. Thereby the trigger circuit 12 is brought into operation to start flashing. At the same time, the transistor Q₂₆ is rendered nonconductive by conductivity of transistor Q₂₄ to make the integration circuit PD₁₀, C₁₂, C₁₃, SW₁₁, Q₂₇ start integration of the light quantity. When the integration output of the light quantity integration circuit exceeds the reference voltage of Zener diode ZD₁, the flash output control signal forming comparator A₁₀ generates a high level output i.e. flash output control signal. This signal actuates the flash terminating circuit 11 through gates AND₁₃, OR₁₁ and AND₁₂ to terminate flashing and also renders transistor Q₂₉ conductive through the gate OR₁₀ of the latch circuit to transmit information of execution of flash output control operation to the unexecuted flash output control detecting circuit. If the flash output control has not been executed, more particularly if no flash output control signal has been generated during the time of transistor Q₂₄ being conductive, then the unexecuted flash output control detecting circuit Q₂₈, Q₂₉, A₁₁ , C₁₄, R₂₁ actuates the warning oscillation circuit R₂₅ -R₂₈, A₁₃, C₁₅ to flicker both of the pilot lamps LED₁ and LED₁₃. Of course, in this independent flash output control mode, TTL flash output control mode selection signal generating circuit Q₂₀, Q₂₁, D₁₀ is inactive and therefore no power is supplied to TTL flash output control circuit 7 on the camera.

(IVA) Operation in the case wherein the electroflash unit is in non-flash output control mode namely entire flash mode with its power source switch being in camera non-interlocking mode and the camera is in electrical shutter control mode or in mechanical shutter control mode with the shutter time being longer than the flash synchronous time:

In this case, the mode selection switch SW₁₀ is in connection with the contact e and keeps the transistor Q₂₈ nonconductive through gate NAND₁₀. Therefore, the unexecuted flash output control detecting circuit is inactive and no warning of unexecuted flash output control is given. By releasing the shutter after lighting of the pilot lamps LED₁ and LED₁₃ on the camera and on the flash unit, entire flashing of the electroflash unit is performed. In other words, the electroflash unit emits flashlight for its maximum flash time.

(VA) Operation in the case wherein the electroflash unit is in camera non-interlocking power source mode and the camera is in mechanical shutter control mode with a shorter shutter time than the flash synchronous time:

In this case, switches SW₆ and SW₇ on camera's side are both closed and the pilot lamp LED₁ on the camera is turned off. Further, the mechanical controlled high speed shutter time detecting comparator A₁₂ has High output, which in turn makes the output of gate NOR₁₀ Low to render transistor Q₃₀ nonconductive thereby bringing the warning oscillation circuit R₂₅ -R₂₈, A₁₃, C₁₅ into operation. Also, the high level output of the comparator A₁₂ opens the gate AND₁₆ through OR₁₃ and closes the gate AND₁₇ through inverter INV₁₄. Therefore, the transistor Q₃₁ is controlled solely by the oscillating output of the warning oscillation circuit so as to flicker the pilot lamp LED₁₃. In this manner, in case that the set shutter time is improper for flashlight photographing, only the pilot lamp LED₁₃ on the electroflash unit's side flickers to give a warning of it.

(VIA) Operation in the case wherein the power source of the electroflash unit is in camera interlocking mode and the camera is in electrical shutter control mode:

In this case, the power source switch SW₁₂ of the flash unit is connected to the contact f to charge the capacitor C₁₇ through resistor R₃₁. This charge current renders transistors Q₃₃ and Q₃₄ conductive to effect a power supply to the circuit of the flash unit, by which the main capacitor C₁₆ is charged. After a definite time determined by the time constant of resistor R₃₁ and capacitor C₁₇ the transistors Q₃₃ and Q₃₄ are rendered nonconductive to block the power supply. During this time, charging of the main capacitor C₁₆ is completed. By conductivity of the power supply transistor Q₁ on the camera, the constant current source 6 supplies the current to the flash unit through terminal T₂, T'₂ to render transistor Q₃₅ conductive. Thereby the capacitor C₁₇ is short-circuited and at the same time the transistor Q₃₃ and Q₃₄ are rendered conductive. Therefore, the power supply to the flash unit is again started. This power supply to the flash unit is maintained so long as the power supply to the camera continues.

When the power supply transistor Q₁ of the camera is rendered nonconductive to cut off the power supply to the camera, the transistor Q₃₅ on the flash unit's side is also rendered nonconductive. By this nonconductivity of Q₃₅ charging of the capacitor C₁₇ is started. The transistor Q₃₃ continues to be conductive for the above mentioned determined time after nonconductivity of Q₃₅. After completing the charging of the main capacitor C₁₆, the transistor Q₃₃ is rendered nonconductive to cut off the power supply to the flash unit.

In this manner, in case that the power source switch SW₁₂ is in camera interlocking mode, the power source of the flash unit is supplied in response to supply of the power source to camera's side. Since, in this case, the switch SW₁₂ has already been in connection with the contact f which always results in charging of the main capacitor, it is possible to start flashlight photographing at once. Moreover, even when the power supply to the camera is cut off, flashlight photographing can be started soon after restarting the power supply to the camera. The reason for this is that even when the power supply to the camera is cut off, the transistor Q₃₃ becomes nonconductive only after the main capacitor C₁₆ has been charged completely.

The resistance value of the resistor R₃₂ mentioned above is determined in the following manner:

When the mode selection switch SW₁₀ is not in the position to select the contact C for TTL flash output control mode, the current supplied to terminal T₂, T'₂ from the camera's constant current source 6 does not flow into TTL flash output control mode selection signal generating circuit Q₂₀, Q₂₁, D₁₀ but flows all into the resistor R₃₂ to render transistor Q₃₅ conductive. The resistance value of the resistor R₃₂ is preset to such value at which the potential at T₂, T'₂ appearing at that time is insufficient to render the transistor Q₉ of the camera nonconductive.

In the combination of the electroflash unit shown in FIG. 2 and the camera shown in FIG. 1, as described above, the flash unit transmits its TTL flash output control mode selection signal to the TTL flash output control mode discriminating circuit of the camera through terminal T'₂, T₂ as a signal in the form of voltage. On the contrary, the signal indicative of the power supply to the camera is transmitted to the power supply control circuit of the flash unit from the camera through terminal T₂, T'₂ as a signal in the form of current. In this manner, the two different signals transmitted between camera and the flash unit through the same terminal T₂, T'₂ are different in form, one of which is current and the other is voltage. This makes it possible to transmit different signals through the same terminal at the same time.

(VIIA) Operation in the case of no power supply to the electroflash unit:

In this case, the power source switch SW₁₂ is in connection with the contact g and therefore no power is supplied to the circuit in the flash unit. Consequently, TTL flash output control mode selection signal generator Q₂₀, Q₂₁, D₁₀ remain inactive even if the mode selection switch SW₁₀ is in connection to the contact C for selecting TTL flash output control mode. Therefore, in this state of the apparatus, no power is supplied to TTL flash output control circuit 7 on the camera, which has an effect to prevent any unnecessary power consumption in the apparatus.

FIG. 3 illustrates another embodiment of an electroflash device according to the present invention. The flash unit of this embodiment has only two modes, namely camera interlocking flash output control mode and independent flash output control mode. It does not have TTL flash output control mode.

Referring to FIG. 3, the flash unit has a mounting foot portion 20 provided with five terminals T₁ ", T₂ ", T₃ ", T_(4") and T₅ ". Since, as noted, the electroflash unit does not have TTL flash output control mode, the contact T_(2") for TTL flash output control is left unconnected after mounting. These five terminals T₁ ", T₂ ", T₃ ", T₄ " and T₅ " are connected to the corresponding five terminals T₁, T₂, T₃, T₄ and T₅ on the camera shown in FIG. 1 respectively in mounting the electroflash unit on the camera.

The power source of the flash unit is designated by 15 at the right-hand edge of FIG. 3. Connected to the power source 15 is a power source switch SW₂₁ having three contacts l, m and n. When the camera interlocking mode contact l is selected by the power source switch SW₂₁, the power supply to the circuit on the flash unit's side is effected interlocking with the power supply on the side of camera. When the contact m is selected, the power supply to the flash unit is cut off at all. When the camera non-interlocking contact n is selected, power is always supplied to the flash unit.

The power source switch SW₂₁ is in link with a flash output control mode selection switch SW₂₀ shown in FIG. 3 at left-hand and lower side of the drawing. When the power source switch SW₂₁ is moved to select contact l, m or n, the mode selection switch SW₂₀ interlocked with the switch SW₂₁ is moved to select camera interlocking flash output control mode contact i, non-connection contact j or independent flash output control mode contact k respectively. Transistors Q₄₂, Q₄₃, Q₄₄, capacitor C₂₁, resistor R₄₆ and diode D₂₆ shown at the right-hand side of the drawing constitute a power supply control circuit. When the power source switch SW₂₁ is connected to contact l, the transistors Q₄₃ and Q₄₄ in the power supply control circuit are rendered conductive for a selected time long determined by the time constant of capacitor C₂₁ and resistor R₄₆ so that the power supply control circuit allows power supply to the circuits on the flash unit. The time mentioned above is preset to that which is equal to or a little longer the charge time to the main capacitor C₁₆. The emitter of transistor Q₄₂ is connected to the contact l and the base is connected to the terminal T₄ " through resistor R₄₅. Its emitter-collector is connected in parallel to the condenser C₂₁. When the camera interlocking mode contact l is selected by the switch SW₂₁ and power is supplied to the camera, a small current flows through the emitter-base of transistor Q₄₂, resistor R₄₅ and terminal T₄ ". Thereby the transistor Q₄₂ is rendered conductive and the capacitor C₂₁ is short-circuited to hold transistors Q₄₃ and Q₄₄ conductive. This small current functions as a flash synchronous shutter time setting signal.

Transistor Q₄₅, resistor R₄₇, capacitor C₂₂ and diode D₂₈ constitute a discharge circuit by which the capacitor C₂₁ is forcedly discharged. When the power source switch SW₂₁ is connected to the contact m, this discharge circuit forcedly discharges the capacitor C₂₁ in a moment. Series-connected to the capacitor C₂₂ is a diode D₂₉ which together with diode D₂₇ and transistor Q₄₄ forms a discharge loop for capacitor C₂₂.

The structures and functions of booster circuit 14, flash discharge tube 13, trigger circuit 12, flash terminating circuit 11, main capacitor C₁₆, diode D₅₀ and charge completion detecting circuit ZD₂, R₂₉, R₃₀ are all the same as those in FIG. 2 and therefore need not be further described.

Resistor R₄₄ and diode D₂₅ supply a small current serving as a flash synchronous time setting equal to the camera through terminal T₄ " when the power source source switch SW₂₁ is in camera uninterlocking mode. More concretely, when the contact l is selected by the power source swith SW₂₁, the small current is supplied to the terminal T₄ " through the emitter-base of transistor Q₄₂ and resistor R₄₅. When the contact n is selected, the small current flows to the terminal T₄ " through resistor R₄₄ and diode D₂₅. D₂₄ is a diode whose anode is connected to that of diode D₂₅ and cathode to the collector of transistor Q₄₄ so that when the contact l is selected, the current flowing through the resistor R₄₄ is directed to transistor Q₄₄ through diode D₂₄ but not to terminal T₄ ". The structures and functions of pilot lamp LED₁₃ and transistor Q₃₁ connected thereto are entirely the same as those in FIG. 2 and therefore need not be further described. Transistor Q₃₂, resistor R₄₃ and diode D₂₃ are connected in series each other to supply a large current to the pilot lamp LED₁ of the camera through terminal T₄ ", T₄ to turn it on when the transistor Q₃₂ is rendered conductive. The functions of NAND gate NAND₁₁, AND gates AND₁₅, AND₁₆, AND₁₇, OR gates OR₁₃, OR₁₄, inverter INV₁₄ and NOR gate NOR₁₀ are all the same as those of the corresponding ones shown in FIG. 2. Also, transistor Q₃₀ and warning oscillation circuit R₂₅ -R₂₈, C₁₅, A₁₃ correspond to those in FIG. 2 in function. Voltage dividing resistors R₂₂ and R₂₃, mechanically controlled high speed shutter time detecting camparator A₁₂ are unexecuted flash output control detector Q₂₈, Q₂₉, A₁₁, C₁₄, R₂₁ also correspond to those in FIG. 2 embodiment. AND gate AND₁₂ and latch circuit OR₁₀, AND₁₁, inverter INV₁₃ have the same functions as the corresponding ones shown in FIG. 2. Function of NAND₂₀ is similar to that of NAND₁₀ in FIG. 2 embodiment. Comparator A₂₄ for forming a flash output control signal and Zener diode ZD₁₀ for generating a reference voltage have the same functions as those of comparator A₁₀ and Zener diode ZD₁ shown in FIG. 2. More particularly, the comparator A₂₄ makes a comparison between the reference voltage of the Zener diode ZD₁₀ and the integrated output from the light quantity integrating circuit PD₂₀, A₂₀, D₂₀ -D₂₂, Q₄₀, Q₄₁, C₂₀. The photo diode PD₂₀ in the integrating circuit receives the flashlight reflected by the object. The differential amplifier A₂₀ has a logarithmic compression diode D₂₀ in its feedback loop and receives, at its one input, the flashlight information from the photo diode PD₂₀ and at its another input the exposure factor information of film sensitivity and diaphragm aperture through mode selection switch SW₂₀ in the manner later described. The transistor Q₄₀ logarithmically expands the output of the differential amplifier A₂₀. In this manner, the transistor Q₄₀ has a collector current corresponding to both the value derived from the photo diode PD₂₀ and data of film sensitivity and diaphragm aperture. The integration condenser C₂₀ carries out integration of the above current in response to nonconductivity of transistor Q₄₁. Constant current source 21, resistors R₄₀ -R₄₂ and variable resistor VR₁ constitute a reference voltage generator. The resistance value of resistor VR₁ is variable depending on the value of film sensitivity and diaphram aperture then set on the side of flash unit. The reference voltage generated from the reference voltage generator 21, R₄₀ -R₄₂, VR₁ is given to the emitter of transistor Q₄₀ by a follower amplifier A₂₁.

To detect it whether or not the set value of film sensitivity and diaphragm aperture in flash output control mode is within the range proper for the mode, there is provided a pair of comparators A₂₂ and A₂₃. When the set value is within the proper range, both of the comparators A₂₂ and A₂₃ become Low in output level. When it is not within the range, either one of amplifiers A₂₂ and A₂₃ becomes High in output. This high level output renders the transistor Q₃₀ nonconductive through OR₂₀ and NOR₁₀ to actuate the warning oscillation circuit R₂₅ -R₂₈, C₁₅, A₁₃. Structures and functions of differentiation circuit C₁₀, R₆₀, R₆₁ and monostable multivibrator circuit Q₂₄, Q₂₅, R₂₀, C₁₁ are the same as those of the corresponding one shown in FIG. 2 and therefore need not be further described.

The manner of operation of the above embodiment will be described hereinafter with reference to FIGS. 1 and 3.

(IB) Operation in the case wherein the electroflash unit is in camera interlocking flash output control mode and therefore its power source switch is also in camera interlocking mode, and the camera is in electrical shutter control mode:

In this case, the mode selection switch SW₂₀ is in connection to contact i and the power source switch SW₂₁ is in connection to contact l on the side of flash unit. On camera's side, the change-over switch SW₁ is closed. By the connection of the power source switch SW₂₁ to contact l, transistors Q₄₃ and Q₄₄ are rendered conductive for a certain time determined by condenser C₂₁ and resistor R₄₆ to supply the power to the circuits on the electroflash unit. Thus, the main capacitor C₁₆ is charged completely. After the lapse of the above determined time, both of transistors Q₄₃ and Q₄₄ are rendered nonconductive to cut off the power supply. When the power supply transistor Q₁ on the camera is rendered conductive to supply the power to the circuits on the camera, a small current flows from the flash unit's power source 15 to the camera through the emitter-base of transistor Q₄₂, resistor R₄₅ and terminal T₄ ", T₄. On the camera's side, a portion of this small current flows into the base of the flash synchronous time setting transistor Q₁₁ through resistor R₁₅ and the remaining portion of the small current flows into the transistor Q₁₀ passing through the pilot lamp LED₁. This small current is preset to such a level at which it can render conductive both of transistor Q₄₂ on the flash unit and transistor Q₁₁ on the camera but can not virtually lighten the pilot lamp on the camera.

By conductivity of this transistor Q₄₂ the capacitor C₂₁ is short-circuited to render transistors Q₄₃ and Q₄₄ conductive to thereby effect power supply to the circuits on the electroflash unit. On the other hand, by conductivity of the transistor Q₁₁ on the camera, the shutter is automatically set to the flash synchronous time.

In this manner, when the camera interlocking mode contact l is selected by the power source switch SW₂₁, a small current automatically flows from camera to flash unit in response to the start of power supply to the camera. Thereby, power supply to the flash unit is started and simultaneously the shutter on the camera is automatically set to the flash synchronous time. As will be understood from the foregoing, this small current performs two different functions at the same time, that is, as a signal for power supply to the flash unit on one hand and as a signal for setting the shutter to flash synchronous time on the other hand. In this connection it is to be understood that when the power supply transistor Q₄₃ is conductive, current is supplied also to resistor R₄₄ but the current flowing through it is essentially directed not to diode D₂₅ but to transistor Q₄₄. The reason for this is that when transistor Q₄₃ is conductive, transistor Q₄₄ also becomes conductive and therefore the current flowing through resistor R₄₄ flows toward the transistor Q₄₄ through diode D₂₄. Essentially no portion of the current flows into the diode D₂₅. This has an effect to prevent such possibility that the current flowing toward terminal T₄ " may become so large as to light up the pilot lamp LED₁ on the camera.

Since the terminal T₂ " is left unconnected, the transistor Q₉ of the camera is conductive and therefore no power is supplied to TTL flash output control circuit 7 on the camera. Transistors Q₉ and Q₁₁ on the camera's side are conductive by which the outputs of inverters INV₂ and INV₃ are changed to High. As a result, the output of AND₃ also becomes High to actuate the amplifier A₃. Therefore, the amplifier A₃ transmits to terminal T₁ the exposure factor information signal of film sensitivity and diaphragm aperture then set at the camera's side. This information signal is applied to one input of operational amplifier A₂₀ through terminal T₁ " and mode selection switch SW₂₀ of the flash unit. In this manner, in case of the camera interlocking flash output control mode, information of exposure factor of film sensitivity and diaphragm aperture set at the camera's side can be introduced into the light quantity integrating circuit on the flash unit in response to the power supply to the camera. The detecting circuit A₂₂, A₂₃ makes a detection as to whether the introduced exposure factor information signal from the mode selection switch SW₂₀ is within the range proper for flash output control mode or not. When it is not within the range, one of the comparators A₂₂ and A₂₃ produces a high level output by which the transistor Q₃₀ is rendered nonconductive through gates OR₂₀ and NOR₁₀ to bring into operation the warning oscillation circuit R₂₅ -R₂₈, A₁₃, C₁₅. The oscillating output from the warning circuit renders the transistor Q₃₂ conductive and nonconductive at a constant frequency. When the transistor Q₃₂ is conductive, a large current flows into the camera through diode D₂₃ and terminal T₄ ". Therefore, with conductivity and nonconductivity of the above transistor Q₃₂ the pilot lamp LED₁ on the camera flickers to give a previous warning of the fact that the set value of film sensitivity-diaphragm aperture is improper for flash output control mode. At this time, the comparator A₁₁ in the unexecuted flash output control detecting circuit Q₂₈, Q₂₉, A₁₁, C₁₄, R₂₁ and the mechanically controlled high speed shutter time detecting comparator A₁₂ are both Low in output level. Therefore, the output of OR₁₃ also becomes Low and the output of AND₁₆ is changed to Low thereby. Consequently, the transistor Q₃₁ is never affected by the output of the warning oscillation and therefore pilot lamp LED₁₃ on the flash unit does not operate for warning in this case. When the above mentioned exposure factor information signal is within the range proper for flash output control mode, the outputs of comparators A₂₂ and A₂₃ are both High in output level and therefore the warning oscillation circuit remains inactive.

When the charge completion detecting circuit ZD₂, R₂₉, R₃₀ detects the completion of charging of the main capacitor C₁₆, the detecting circuit renders transistor Q₃₂ conductive through gates NAND₁₁ and AND₁₅ on one hand and renders transistor Q₃₁ conductive through gates AND₁₇ and OR₁₄ on the other hand. On conductance of these transistors Q₃₂ and Q₃₁, the pilot lamp LED₁ on the camera as well as the pilot lamp LED₁₃ on the flash unit light up to indicate that the apparatus is now ready for flashing.

When the shutter is fully opened, a signal is transmitted to the differentiation circuit C₁₀, R₆₀, R₆₁ from the camera through terminal T₃, T₃ ". In response to the signal, the differentiation circuit triggers the monostable circuit Q₂₄, Q₂₅, R₂₀, C₁₁. Thereby the transistor Q₂₄ is rendered conductive and continues to be conductive for a determined time (which time is equal to or a little longer than the maximum flash duration time of the electroflash unit). In response to conductivity of this transistor, the trigger circuit 12 makes the flash discharge tube 13 start flashing. Also, conductivity of this transistor Q₂₄ renders transistor Q₄₁ nonconductive through diodes D₂₁ and D₂₂ at the same time to start the light quantity integrating operation of the capacitor C₂₀. The photo diode PD₂₀ receives the flashlight reflected by the object and the capacitor C₂₀ carries out integration of the received flashlight. As soon as the output of integrated flashlight has reached the level of the reference voltage by Zener diode ZD₁₀, the output of the comparator A₂₄ is reversed from Low to High to produce a flash output control signal. This signal changes the output of AND₁₂ to High by which the flash terminating circuit 11 is actuated. Thus, flashing is terminated. At the same time, High output of AND₁₂ renders transistor Q₂₉ conductive through gate OR₁₀ to discharge the capacitor C₁₄ in a moment.

As understood from the above, when a flash output control operation is executed in the manner described above, the unexecuted flash output control detecting circuit Q₂₈, Q₂₉, A₁₁, C₁₄, R₂₁ does not virtually operate the warning oscillation circuit R₂₅ -R₂₈, A₁₃, C₁₅, which is the same as the first embodiment shown in FIG. 2. On the contrary, when the light quantity integration voltage of the capacitor C₂₀ has not reached the reference voltage given by Zener diode ZD₁₀, there is produced no flash output control signal. Therefore, like the first embodiment shown in FIG. 2, the unexecuted flash output control detecting circuit actuates the warning oscillation circuit for a predetermined time. Therefore, both of the pilot lamps LED₁ and LED₁₃ flicker indicating that no flash output control has been executed.

By nonconductivity of the power supply transistor Q₁ of the camera, the transistor Q₁₀ series-connected to the pilot lamp LED₁ is also rendered nonconductive which in turn restricts the small current flowing through terminal T₄ ", T₄ from the flash unit. Therefore, the transistor Q₄₂ on the flash unit becomes nonconductive and charging of capacitor C₂₁ is started. When the charge voltage on the capacitor reaches a predetermined level, transistors Q₄₄ and Q₄₃ are rendered nonconductive to cut off the power supply to the flash unit. In this manner, when the power supply to the camera is cut off, the power supply to the flash unit is also automatically cut off interlocking with it. Like the first embodiment shown in FIG. 2, at this time the transistor Q₄₅ continues to be conductive for the predetermined time after the stop of power supply to the camera so as to complete charging of the main capacitor C₁₆. Therefore it is possible to carry out flashlight photographing immediately after restarting the power supply to the camera.

In the above embodiment, transistor Q₄₂ of the flash unit is controlled according to the state of transistor Q₁₀ on the camera's side. The latter mentioned transistor Q₁₀ is in turn controlled by the output of camera's battery checking comparator A₂. Therefore, if the voltage of camera's power source drops under a predetermined value, then the transistor Q₁₀ becomes nonconductive and there is no power supply to the flash unit even when the power supply transistor Q₁ of the camera is rendered conductive. This prevents flashlight photographing in camera interlocking flash output control mode under the condition of too low a voltage of the camera's power source. When the source voltage has dropped so much, problems in operation occur. For example, the electrical shutter control operation can not correctly be carried out and/or the exposure factor information signal can not correctly be transmitted. In such case, the above arrangement inhibits flashlight photographing in camera interlocking flash output control mode.

Conductivity of the above transistor Q₄₄ discharges the capacitor C₂₂ of the forced discharge circuit C₂₂, R₄₇, Q₄₅ through diodes D₂₇ and D₂₉. On change-over of the power source switch SW₂₁ from contact l to m, charging of the capacitor C₂₂ is started. The transistor Q₄₅ becomes conductive for a certain determined time so that the capacitor C₂₁ is discharged through the transistor Q₄₅ and diode D₂₈ in a moment. Thus, the capacitor gets in the state prepared for the next connection of power source switch SW₂₁ to the contact l.

(IIB) Operation in the case wherein the electronic flash unit is in camera interlocking flash output control mode and therefore its power source switch is in camera interlocking mode but the camera is in mechanical shutter control mode with flash synchronous time:

In this case, on the side of flash unit, switches SW₂₀ and SW₂₁ are connected to contacts i and l respectively. On the side of camera, its change-over switch SW₁ is opened, SW₆ is closed and SW₇ is opened. Since the transistor Q₁₀ is short-circuited by closing of switch SW₆, a small current flows into the pilot lamp LED₁ and the above switch SW₆ on the camera from the flash unit through terminal T₄ ", T₄ on one hand. On the other hand, it flows into the base of transistor Q₁₁. Thereby the transistor Q₄₂ on the flash unit and therefore also the transistor Q₄₃ are rendered conductive. Thus, power supply to the flash unit is effected.

In the embodiment shown in FIG. 3, when the power source switch is in camera interlocking mode, power supply to the electroflash unit is effected naturally interlocking with the power supply to the camera in the manner described above. Moreover, in case of no power supply to the camera, the flash unit can be power-supplied interlocking with the selection of mechanical shutter control mode with flash synchronous time.

In the event there is no power supply to the camera, the transistor Q₅ of the camera is nonconductive and the amplifier A₃ is inactive. As a result, the positive potential of power source 15 is applied to the contact i of the flash unit through resistor R₄₈. Thereby, the comparator A₂₂ in the exposure factor suitability detecting circuit becomes High in output level. This high level output renders transistor Q₃₀ nonconductive through gates OR₂₀ and NOR₁₀ to actuate the warning oscillation circuit R₂₅ -R₂₈, A₁₃,C₁₅. The oscillating output from the warning circuit flickers the camera's pilot lamp LED₁ in the same manner as above to previously warn the operator that there is no introduction of exposure factor information signal from the side of camera and threrefore the camera interlocking flash output control mode is impossible. Such previous warning is given also in the case wherein the camera is not provided with the function of generating the above mentioned exposure factor infromation signal. An example of such case is a camera as shown in FIG. 1 but without amplifier A₃. In this case, the terminal T₁ is left unconnected or connected to ground. The first mentioned unconnected state and the second mentioned grounded state can be detected by the comparators A₂₂ and A₂₃ of the introduced exposure factor suitability detecting circuit respectively. Therefore, a previous warning is given in a manner similar to the above.

(IIIB) Operation in the case wherein the elctronic flash unit is in independent flash output control mode and therefore its power source switch is in camera non-interlocking mode:

In this case, since the power source switch SW₂₁ is in connection with the contact, power supply to the flash unit is carried out independently of that to the camera. On the other hand, the mode selection switch SW₂₀ is in connection with the contact k. Therefore, the resultant value from film sensitivity and diaphragm aperture then set at the side of flash unit is introduced into the operational amplifier A₂₀ of the light quantity integrating circuit from the variable resistor VR₁. Naturally the outputs of the comparators A₂₂ and A₂₃ are both High under the condition of the voltage generated by the variable resistor VR₁.

When the camera is in electrical shutter control mode, the shutter is automatically set to the flash synchronous time by the small current flowing into the camera from the flash unit through resistor R₄₄, diode D₂₅ and terminal T₄ ", T₄. With the opening of the shutter, the flash unit start flashing. Flash output control operation is performed on the basis of both the incident light to the photo diode PD₂₀ in the flash unit and the exposure value of film sensitivity-diaphragm aperture preset at the side of flash unit. If no flash output control has been executed, then the unexecuted flash output control detecting circuit detects it and actuates the warning oscillation circuit to flicker the pilot lamp LED₁ on the camera and the pilot lamp LED₁₃ on the flash unit.

(IVB) Operation in the case wherein the camera is in mechanical shutter control mode with shutter time shorter than flash synchronous time:

In this case, switches SW₆ and SW₇ of the camera are both closed. Therefore, the pilot lamp LED₁ on the camera remains always extinct. The high speed time detecting comparator A₁₂ of the flash unit detects the closed state of above two switches SW₆ and SW₇ and flickers the pilot lamp LED₁₃ on the flash unit through the warning oscillation circuit to previously warn that flashlight photographing is improper to the case.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, obviously various modifications and variations are possible in light of the above teachings. For example, there may be used also display elements other than the light emitting elements shown in the above embodiments for giving various warnings described above. Examples of other useful elements are optical display elements and acoustic elements. In the above embodiments, elements such as pilot lamps provided for displaying that the apparatus is ready for flashing, have been used also for warning. However, there may be provided display elements for warning only.

A light quantity integration circuit and/or TTL flash output control signal forming comparator for TTL flash output control mode may be provided also within the electroflash unit itself.

Also, a discrimination circuit for judging whether or not the value of film sensitivity introduced into the camera in TTL flash output control mode is proper may be provided also within the electroflash unit. Furthermore, this discrimination circuit and the detection circuit provided on the side of flash unit for detecting the suitability of introduced exposure factors may be formed in common.

In the above embodiments, only the electronic flash unit has been shown as an attachment to camera. However, the application form of the present invention is not limited to such combination only. Other attachment such as a motor drive may be used in accordance with the present invention.

As will be understood from the foregoing, the present invention has many advantages over the prior art. A plural number of signals can be transmitted and received between a camera and a device attached to it without any interference. It provides a signal transmitting and receiving apparatus which is simple in structure and inexpensive. 

I claim:
 1. An attachment unit attachable to a camera unit including a camera unit terminal, current signal generating means for generating a signal in a form of current and a voltage detection circuit, said attachment unit comprising;an attachment unit terminal adapted for connection to said camera unit terminal, voltage signal generating means for generating on said attachment unit terminal a signal in a form of voltage, which is in turn detected by said voltage detection circuit, even when said current signal generating means is generating the current signal, and a current detection circuit for detecting said current signal on said attachment unit terminal.
 2. An attachment unit according to claim 1, wherein said voltage signal generating means includes a transistor having a control electrode connected to said attachment unit terminal, said voltage signal being generated on said control electrode and said transistor being rendered conductive by the current signal flowing through said control electrode, the conduction of said transistor serving as said detection signal.
 3. An attachment unit attachable to a camera unit including a camera unit terminal, current signal generating means for generating on said camera unit terminal a signal in the form of current and a voltage detection circuit, said attachment unit comprising;an attachment unit terminal adapted for connection to said camera unit terminal, and voltage signal generating means for generating on said attachment unit terminal a signal in the form of voltage, which is in turn detected by said voltage detection circuit, and for detecting said current signal through said attachment unit terminal to produce a detection signal.
 4. An attachment unit according to claim 3, wherein said voltage signal generating means includes a transistor having a control electrode connected to said attachment unit terminal, said voltage signal being generated on said control electrode and said transistor being rendered conductive by the current signal flowing through said control electrode, the conduction of said transistor serving as said detection signal.
 5. In an electronic flash unit attachable to a camera, said camera comprising:means for generating, on a first terminal an exposure factor signal related to a set value of exposure factors; measuring means for measuring the light transmitted through a lens of the camera and for generating a measured light signal on said first terminal after the end of said exposure factor signal; and means for generating a switchover signal on a second terminal, and said electronic flash unit comprising:means for generating flashlight; third and fourth terminals for connection to said first and second terminals respectively; responding means operable in response to said exposure factor signal; terminating means for terminating flashlight emission according to said measured light signal; and switchover means for receiving said switchover signal from said fourth terminal and, before reception of said switchover signal, for transmitting the signal from said third terminal to said responding means and, after the reception of said switchover signal, for transmitting the signal from said third terminal to said terminating means.
 6. An electronic flash unit according to claim 5 which further comprises trigger means for actuating said flashlight generating means in response to said switchover means.
 7. In a combination of a camera unit and an attachment unit, said combination comprising:a terminal for connecting the attachment unit to the camera unit; current signal generating means provided on one of the units for generating a signal in a form of current; voltage signal generating means provided on the other unit for generating a signal in a form of voltage even when said current signal generating means is generating the current signal; a current detection circuit provided on said other unit for detecting said current signal on said terminal; and a voltage detection circuit provided on said one unit for detecting said voltage signal on said terminal.
 8. A combination as set forth in claim 7 wherein said attachment unit is an electronic flash unit.
 9. A combination as set forth in claim 7 wherein said current signal generating means is a current source and said voltage signal detection circuit is connected to an output of said current source with the connection point being connected to said terminal, and wherein said voltage signal generating means, when it receives the current from said current source, keeps said terminal at a predetermined constant potential which serves as said voltage signal.
 10. In a combination of a camera and an electronic flash unit attachable thereto, the camera comprising first and second terminals; information signal generating means comprising a first circuit for generating on the first terminal before a shutter releasing operation a first information signal corresponding to a set value of exposure factors, and a second circuit for generating on the first terminal after the shutter releasing operation a second information signal corresponding to the light transmitted through a lens of the camera; and switchover signal generating means for generating a switchover signal on the second terminal in response to the shutter releasing operation, the information signal generating means generating the second information signal prior to the generation of the switchover signal; and said electronic flash unit comprising third and fourth terminals for connection to said first and second terminals, respectively; first responding means comprising warning means for generating a warning signal, and means for actuating said warning means in accordance with the first information signal when said set value of exposure factors is outside of a predetermined range; second responding means comprising means for terminating flashing of the electronic flash unit in response to said second information signal; and a switchover circuit for selectively transmitting to the first and second responding means said first and secnd information signals through the connected first and third terminals in accordance with said switchover signal being transmitted through the connected second and fourth terminals.
 11. A combination according to claim 10, wherein said switchover signal generating means generates said switchover signal in response to a shutter opening operation, and wherein said electronic flash unit includes trigger means for causing said electronic flash unit to start flashing in response to said switchover signal.
 12. An electronic flash unit attachable to a camera, said camera comprising first and second terminals; means for generating, on said first terminal, a first information signal before a shutter releasing operation and a second information signal after the shutter releasing operation; and synchronous means for producing on the second terminal a synchronous signal in response to a shutter operation; said electronic flash unit comprising:third and fourth terminals adapted for connection to said first and second terminals, respectively; first and second responding means; means for emitting; flashlight in response to said synchronous signal being transmitted through the connected second and fourth terminals; and means responsive to said synchronous signal for selectively transmitting to said first and second responding means said first and second information signals through the connected first and third terminals.
 13. An electronic flash unit according to claim 12, wherein said second responding means includes means for terminating flashlight emission in response to said second information signal.
 14. A combination of a camera unit and an attachment unit attachable thereto, said combination comprising:a terminal for connecting the attachment unit to the camera unit; current signal generating means provided on one of the units for generating on said terminal a signal in the form of current; voltage signal generating means provided on the other one of the units for generating on said terminal a signal in the form of voltage and for detecting said current signal through said terminal to produce a detection signal; and a voltage detection circuit provided on said one unit for detecting said voltage signal on said terminal.
 15. A combination according to claim 14, wherein said voltage signal generating means includes a transistor having a control electrode connected to said terminal, said voltage signal being generated on said control electrode and said transistor being rendered conductive by the current signal flowing through said control electrode, the conduction of said transistor serving as said detection signal.
 16. In a combination of a camera unit and an attachment unit attachable thereto, one of said units comprising:a first terminal; first mode selection means for alternately selecting a first mode and a second mode; transistor means including a control electrode connected to the first terminal; and means for producing a voltage signal on said control electrode when said first mode selection means selects said first mode; the other of said units comprising: a second terminal connectable to said first terminal; second mode selecting means for alternately selecting a third mode and a fourth mode; means connected to said second terminal for detecting said voltage signal transmitted through said connected first and second terminals; and mean for permitting a current signal to flow through said control electrode and said connected first and second terminals to thereby render said transistor means conductive when said second mode selecting means selects said third mode.
 17. A combination according to claim 16, wherein said one unit further comprises means responsive to the conduction of said transistor means for setting said one unit in an operating condition suitable to said third mode.
 18. A combination according to claim 17, wherein said voltage signal detecting means sets said other unit in an operating condition suitable to said first mode in response to said voltage signal.
 19. A combination according to claim 18, wherein said one unit is the attachment unit and the other unit is the camera unit. 